For safety and environment protection, there has conventionally been a demand for reducing an amount of a fuel evaporated from the wall or connection of a fuel tank or attachments thereto. In the automobile industry, for example, a resin fuel tank having a single layer structure and being made of high-density polyethylene (which may be called “HDPE resin”) by the blowing method is in widespread use as a fuel tank body for automobiles. In the case of a resin fuel tank, various methods have been developed in order to reduce a permeation amount of a fuel from the tank body, for example, a method of sulfonating the resin (SO3 treatment, Japanese Patent Publication No. 23914/1971), a method of treating with fluorine (F2 treatment), a method of producing a blow molding having a multilayer structure with a barrier resin (Japanese Patent Publication No. 49989/1980) and a method of dispersing, in the form of flakes, a barrier resin such as polyamide in a continuous matrix phase of polyethylene (Japanese Patent Publication No. 14695/1985).
In particular, a resin fuel tank of a multilayer structure having an HDPE resin layer formed as the outer layer of the fuel tank and at the same time, having, on the inside of the fuel tank, an inner layer made of a polyamide resin or ethylene/vinyl alcohol copolymer excellent in barrier properties against a fuel gas has come to be popular recently.
Even if a permeation amount of a fuel from a fuel tank body is reduced by the above-described method, permeation from various parts (ex. valves) attached to the fuel tank cannot be negligible and becomes an obstacle to reduction in the total evaporation amount from the fuel parts.
Although various parts attached to a fuel tank are, in order to attain sufficient adhesion strength with the fuel tank, made of high-density polyethylene similar to the fuel tank, this high-density polyethylene is inferior in fuel permeation resistance.
It was therefore investigated to produce various parts for a fuel tank by using a polyamide resin excellent in fuel permeation resistance. If so, however, inferior adhesion between polyethylene and polyamide caused such a problem as insufficient adhesion strength with the fuel tank.
Production of a fuel part having a multilayer structure made of a polyethylene layer, an adhesive layer and a polyamide layer was investigated, but it was accompanied with the problem that when the part was brought in contact with gasoline or gasohol, swelling of the polyethylene layer and adhesive layer occurs and it induces lowering in interlayer adhesion strength between the polyethylene layer and polyamide layer, leading to generation of interlayer peeling or cracks.
In Japanese Patent No. 2715870, described is use of a glass-fiber-reinforced polyamide resin for a part of a polyethylene joint for joining a flexible conduit to a polyethylene tank for the purpose of preventing fuel leakage due to creep of polyethylene at the connection of a tube such as coupler.
In general, it has been regarded that polyamide resins have low gasoline permeability, are almost free from swelling when in contact with gasoline and hardly cause lowering in physical properties or dimensional change. In recent years, however, gasohol, that is, gasoline mixed with 10 to 15 wt. % of methanol or ethanol based on the total amount of the mixture has frequently been used. In a direct fuel-injection engine, fuel temperature in the tank increases even to 60 to 80° C. due to some return of the fuel. It has been found that the polyamide swells with methanol or ethanol in gasohol at such high using temperatures and in particular, an elastic modulus in a direction perpendicular to the orientation reinforcing direction of glass fibers shows a marked decrease, leading to a creeping phenomenon.
This suggests that not only a simple substance of polyamide but also fiber-reinforced polyamide cannot be used for a structural member in contact with a gasohol fuel.